1. Field of the Invention
The present invention relates to techniques for determining formation fluid properties. More particularly, the invention relates to determination of the oil viscosity and continuous gas oil ratio (GOR) from nuclear magnetic resonance (NMR) logs.
2. Background of the Related Art
The oil and gas industry has developed various tools capable of determining formation fluid properties. For example, borehole fluid sampling and testing tools such as Schlumberger's Modular Formation Dynamics Testing (MDT™) Tool can provide important information on the type and properties of reservoir fluids in addition to providing measurements of reservoir pressure, permeability, and mobility.
These various tools may perform measurements of the fluid properties downhole, using sensor modules on board the tools. Alternatively, these tools can withdraw fluid samples from the reservoir that can be collected in bottles and brought to the surface for analysis. The collected samples are routinely sent to fluid properties laboratories for analysis of physical properties that include, among other things, oil viscosity, GOR, mass density or American Petroleum Institute (API) gravity, molecular composition, H2S (hydrogen sulfide), asphaltenes, resins, and various other impurity concentrations. However, the laboratory data may not be useful or relevant because it is discrete (non-continuous) and receiving the data after analysis is too late for real-time decision-making at the wellsite.
Techniques have been developed in analyzing formation fluid properties using NMR data. For example, SPE 75325 paper “A New NMR Method of Fluid Characterization in Reservoir Rocks: Experimental Confirmation and Simulation Results” by Freedman et al. presents techniques to evaluate water, oil, and gas in reservoirs; the paper “NMR Properties of Reservoir Fluids” by Hirasaki et al., published in AAPG 2002, describes GOR calculation from crude oil NMR data; SPE 84478 paper “Planning and Interpreting NMR Fluid-Characterization Logs” by Cao Minh et al., SPE 84482 paper “A Next-Generation Wireline NMR Logging Tool” by Depavia et al., and SPE 90564 paper “Saturation and Viscosity From Multidimensional Nuclear Magnetic Resonance Logging” by Heaton et al. describe techniques of using diffusion-relaxation time maps derived from NMR data in analyzing formation fluid properties.
Several U.S. patents and Patent Publication disclose methods and apparatus for making NMR measurements in a borehole on fluid samples withdrawn from earth formations. For example, U.S. Pat. No. 6,346,813 B1 issued to Kleinberg discloses an NMR module on the flowline of the wireline formation testers (WFT) (e.g., MDT™ by Schlumberger Technology Corp. (Houston, Tex.)) for determining different fluid properties from magnetic resonance signals; U.S. Pat. No. 6,107,796 issued to M. Prammer discloses apparatus and methods for determining the level of contamination in a formation crude oil sample that may be contaminated by oil-based mud (OBM) filtrate; U.S. Pat. No. 6,111,408 issued to Blades et al. discloses methods and apparatus for measuring the relaxation times (T1 and T2) and the diffusion coefficients (D) of fluids in an NMR module of a fluid sampling tool; U.S. Pat. No. 6,346,813 B1 issued to Kleinberg discloses an NMR module for characterizing fluids in a fluid sampling and testing tool, such as the MDT™ tool; and U.S. Patent Publication No. 2004/0169511A1 by Cao Minh et al. describes a method for interpreting multi-dimensional NMR data.
Despite the development and advancement of various NMR apparatus and methods for determining formation fluid properties based on acquiring formation fluid samples from inside the wellbore, GOR correlation between calculation and measurement has been poor and there remains a need to provide techniques capable of providing a continuous log of formation fluid types and properties in a non-invasive manner without resulting in unnecessary or insufficient sampling.